JPH09205388A - Adaptive noise elimination automobile telephone system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the S/N and to decrease the sound echo by interposing an adaptive filter whose characteristic is changed adaptively to each signal path. SOLUTION: A voice signal is amplified by an amplifier 2 and converted into a digital signal by an A/D converter 3 and given to an adder 4 after passing through an adaptive high-pass filter 14. The adder 4 adds an output of the filter 14 and an output of an adaptive digital filter(ADF) 12 and gives the sum being an analog signal to a radio equipment 6. On the other hand, the reception signal is converted into a digital signal by an A/D converter 8, multiplied by a multiplier 13 and part of the signal passes through an adaptive high- pass filter 15 and the low frequency component is eliminated and the result is given to the ADF 12 and the other part is converted into an analog signal by a D/A converter 9 and amplified by an amplifier 10 and outputted as a voice from a speaker 11. Then the characteristics of the adaptive filters 14, 15 are changed adaptively depending on a transmission signal outputted from the adder 4, that is, the quantity of an echo estimate error of the ADF 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise elimination technique for a car telephone device, and more particularly to a car telephone device capable of suitably eliminating noise in response to a change in noise accompanying a change in the traveling speed of a vehicle.

[0002]

2. Description of the Related Art Conventionally, in a hands-free car telephone device that enables a telephone call while the vehicle is running, external noise and sound from a speaker are generated because the driver and the microphone are far from each other. Easy to get into the microphone,
This is a cause of poor call performance. Therefore, an automobile telephone device having a function of removing noise input from such a microphone has been proposed. FIG. 3 is a block diagram showing an example of a conventional car telephone device of this type. In the figure, when a speaker M who makes a hands-free call inputs his or her voice into the microphone 1 for transmission,
The audio signal is amplified by the amplifier 2, and the A / D converter 3
Is converted into a digital signal and input to the adder 4. The adder 4 adds the output of the A / D converter 3 and the output of an adaptive digital filter (ADF) 12 described later,
The analog signal is converted by the D / A converter 5 and then input to the wireless device 6. The wireless device 6 transmits the input talker signal of the speaker to the partner speaker via the antenna 7.

On the other hand, the received signal from the other speaker received by the antenna 7 and the radio 6 is converted into a digital signal by the A / D converter 8, and a part of the digital signal is input to the ADF 12. , The other part is converted into an analog signal in the D / A converter 9, and then the amplifier 10
Is amplified by and is output as sound from the speaker 11.

The ADF 12 is an adaptive digital filter for generating an estimated echo, and a transversal type is generally used, and the number of taps is determined according to the impulse response length of the acoustic coupling between the speaker 11 and the transmission microphone 1. . In addition, the adaptive control of the ADF 12 is generally performed by a learning identification method or the like so that the mean square value of the estimation error is minimized. An estimated echo is generated based on the reception signal digitally converted by the A / D converter 8, and the estimated echo is added (subtracted) to the transmission signal input from the transmission microphone 1 in the adder 4 and transmitted. It functions as an acoustic echo canceller that removes the acoustic echo component of the speech signal. With the function of the acoustic echo canceller, it is possible to cancel the acoustic echo generated when the received voice from the speaker 11 wraps around the transmitting microphone 1. This acoustic echo not only hinders dialogue, but, in the worst case, causes an oscillation phenomenon called howling. An example of such a hands-free automobile telephone device is disclosed in Japanese Patent Application Laid-Open No. 4-108246.

[0005]

In such a conventional automobile telephone device, the following problems occur.
The first problem is that external noise such as engine noise, wind noise of the body, and ground contact noise of tires enter the inside of the vehicle while the vehicle is traveling. When these external noises are input to the microphone for transmission, A as an acoustic echo canceller is used.
The echo estimation error in the DF (hereinafter referred to as the echo estimation error) becomes large, and the acoustic echo that wraps around from the speaker 9 to the transmitting microphone 1 as described above is not sufficiently canceled.

The second problem is that a high-efficiency voice coding system is adopted in a digital radio telephone,
When external noise is input together with voice, the call quality is extremely deteriorated. A high-pass filter is usually inserted to reduce this, but when the noise level is low, the quality of the transmitted voice is rather deteriorated. Furthermore, the third problem is that, in the vehicle, the clarity of the received voice reproduced from the speaker 9 decreases as the external noise level increases.

An object of the present invention is to provide an adaptive noise-removing mobile telephone device which is not easily influenced by external noise and which is excellent in speech quality.

[0008]

According to the present invention, there is provided an acoustic echo canceller for generating an estimated echo based on a received signal, adding the estimated echo to a transmitted signal and removing an acoustic echo component of the transmitted signal. In the adaptive noise-removing mobile telephone device, the characteristics adaptively change depending on the magnitude of the error of the estimated echo in each signal path of the reception signal input to the acoustic echo canceller and the transmission signal to which the estimated echo is added It is characterized in that each adaptive filter is inserted. As the adaptive filter, adaptive high-pass filters having the same characteristics are used. Further, it is preferable to include means for adaptively controlling the reception volume based on the characteristic change of the adaptive filter.

For example, the car telephone system of the present invention includes a transmission system for inputting a voice input from a microphone as a transmission signal to a car telephone radio, and a voice output from the radio and output from a speaker as voice. A receiving system, an adaptive digital filter as an acoustic echo canceller interposed between the transmitting system and the receiving system, an adder for adding the transmitting signal and the output of the adaptive digital filter, and the transmitting system In the system, there is provided a first adaptive high-pass filter inserted in the preceding stage of the adder, and a second adaptive high-pass filter inserted between the receiving system and the adaptive digital filter. The second adaptive high-pass filter is configured such that the characteristic is adaptively changed based on the output level of the adder. A first arithmetic unit for calculating an effective value of a signal input to the second adaptive filter; a second arithmetic unit for calculating an effective value of a signal output from the second adaptive filter; A comparison arithmetic unit for calculating a multiplication coefficient by performing a comparison arithmetic operation on the outputs of the first and second arithmetic units, and a multiplier inserted in the reception system for controlling the level of the reception signal based on the multiplication coefficient. I have it.

[0010]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention, and the configuration of each system of the transmission signal and the reception signal is almost the same as that of the conventional hands-free automobile telephone device shown in FIG. That is, the speaker M making a hands-free call
When his voice is input to the microphone 1 for transmission, the audio signal is amplified by the amplifier 2, converted into a digital signal by the A / D converter 3, and further passed through the adaptive high-pass filter 14 as described later. , Are input to the adder 4. The adder 4 adds this output to the output of the ADF 12, converts it into an analog signal in the D / A converter 5, and inputs the analog signal to the wireless device 6. The wireless device 6 transmits the input talker signal of the speaker to the partner speaker via the antenna 7.

On the other hand, the received signal from the other speaker received by the antenna 7 and the wireless device 6 is converted into a digital signal by the A / D converter 8 and further multiplied by a multiplier 13 as described later. Then, a part of the digital signal is transmitted through the adaptive high-pass filter 15 to remove low-frequency components, and then input to the ADF 12.
The other part is converted into an analog signal in the D / A converter 9, amplified by the amplifier 10, and output as sound from the speaker 11.

The ADF 12 is an adaptive digital filter for generating an estimated echo, which is the same as before, a transversal type is used, and the number of taps is an impulse of acoustic coupling between the speaker 11 and the transmission microphone 1. It is determined according to the response length, and its adaptive control is generally performed by a learning identification method or the like so that the mean square value of the estimation error is minimized.

Here, the ADF 12 includes the multiplier 13
The signal from which the low-frequency component is removed by the adaptive high-pass filter 15 is input. Further, the input signal from the multiplier 13 input to the adaptive high-pass filter 15 and the output signal of the adaptive high-pass filter 15 are input to the individual effective value calculators (RMS) 16 and 17, respectively. The output of is subjected to a predetermined calculation in the comparison calculator 18, and the multiplication coefficient of the multiplier 13 is controlled according to the calculation result.

The adaptive high-pass filters 14 and 1
5 have the same characteristics, and are configured as an adaptive high-pass filter whose characteristics are adaptively changed depending on the transmission signal output from the adder 4, that is, the echo estimation error of the ADF 12.

Next, the operation of the apparatus shown in FIG. 1 will be described in detail. FIG. 2A shows an echo estimation error characteristic in the adaptive digital filter according to a level change of traveling noise (hereinafter referred to as vehicle interior noise) entering the vehicle, and FIG.
FIG. 6B is a diagram schematically showing the transmission characteristics of the adaptive high-pass filter associated therewith. Here, in general, the noise in the vehicle depends on the traveling speed, and the frequency component of the noise has a larger energy density in the lower range, and its spectral characteristic is approximately 10 dB /
It is oct. In addition, the fluctuation of the spectral characteristics due to the difference in the measurement conditions such as the type of vehicle and the traffic volume is 6-12 dB / o
ct.

When the vehicle speed is about 50 km / h or less, the noise level inside the vehicle is low, so the error in echo estimation by the ADF 12 is smaller than the first threshold level, and the actual echo suppression amount is 20 to 25 dB. The characteristics of the adaptive high-pass filters 14 and 15 in such a state are the mildest, for example, a cutoff frequency of 300 Hz and a cutoff characteristic of 6
Since it is dB / oct, there is no deterioration in the transmission voice quality due to the adaptive high pass filter.

On the other hand, when the vehicle exceeds 60 km / h, the noise level in the vehicle gradually increases and the echo estimation error exceeds the first threshold level. From such a state, the characteristics of the adaptive high-pass filter adaptively change. For example, the cutoff characteristic is 6 dB / oct to 48 dB /
The cutoff frequency gradually increases to oct, and at the same time, the cutoff frequency gradually shifts from 300 Hz to a maximum of 600 Hz to the high frequency side. As described above, since the vehicle interior noise has a large low frequency energy density, such a characteristic change of the adaptive high pass filter improves the transmission S / N.

On the contrary, when the traveling speed decreases and the in-vehicle noise level decreases, the echo estimation error becomes smaller than the first threshold level and further smaller than the second threshold level. Even in such a state, the characteristics of the adaptive high pass filter adaptively change. The steep cutoff characteristic gradually becomes gradual, and at the same time, the cutoff frequency also gradually shifts to the low frequency side.

When the echo estimation error is between the first threshold level and the second threshold level, the characteristic of the adaptive high pass filter does not change. That is, the servo loop is formed so that the echo estimation error is always between the first threshold level and the second threshold level.

On the other hand, the volume level of the received voice is determined by the multiplier 13
It is changed by the multiplication coefficient given to. The input signal and output signal of the adaptive high pass filter 15 are RMS 16 and
Each effective value input to 17 and calculated here is output to the comparison calculator 18. The comparison calculator 18 calculates the ratio of the two effective values and generates a multiplication coefficient. The multiplication coefficient becomes 1 when the cutoff characteristic of the adaptive high-pass filter is the gentlest, and increases as the cutoff characteristic becomes steeper.
That is, when the in-vehicle noise level rises, the volume level of the received voice increases and the intelligibility of the received voice improves. The maximum value of the multiplication coefficient is specified. For example, the maximum value is 2
Then, the receiving volume automatically changes in the range of 0 to 6 dB.

[0021]

As described above, the car telephone system of the present invention has the following structure in each signal path of the reception signal input to the acoustic echo canceller provided between the transmission and reception systems and the transmission signal to which the estimated echo is added. , Since the adaptive filters whose characteristics are adaptively changed according to the magnitude of the error of the estimated echo are inserted, the echo estimation error changed by following the level fluctuation of the vehicle interior noise accompanying the change of the vehicle speed etc. The characteristics of the adaptive filter are changed, which causes S
/ N is improved, acoustic echo is reduced, and conversation quality and transmission clarity are improved. Further, by providing the means for adaptively controlling the listening volume based on the characteristic change of the adaptive filter, even if the in-vehicle noise level rises, the intelligibility of the listening is not extremely deteriorated.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a diagram showing a characteristic of an echo estimation error based on a change in an in-vehicle noise level and a characteristic of an adaptive high pass filter.

FIG. 3 is a block diagram of an example of a conventional hands-free car telephone device.

[Explanation of symbols]

 1 Transmitting microphone 2,11 Amplifier 3,8 A / D converter 4 Adder 5,9 D / A converter 6 Radio 12 ADF (adaptive digital filter) 13 Multiplier 14,15 Adaptive high-pass filter 16,17 RMS (RMS calculator) 18 Comparison calculator

Claims (6)

[Claims] 1. A car telephone apparatus comprising an acoustic echo canceller for generating an estimated echo based on a received signal and adding the estimated echo to a transmitted signal to remove an acoustic echo component of the transmitted signal. An adaptive filter whose characteristics are adaptively changed depending on the magnitude of the error of the estimated echo is inserted in each signal path of the reception signal input to the echo canceller and the transmission signal to which the estimated echo is added. A noise reduction car telephone system characterized by the following. 2. The adaptive noise canceling mobile telephone apparatus according to claim 1, wherein the adaptive filters are adaptive high-pass filters having the same characteristics. 3. The adaptive noise canceling mobile telephone apparatus according to claim 1 or 2, further comprising means for adaptively controlling a received sound volume based on a characteristic change of the adaptive filter. 4. A transmission system for inputting a voice input from a microphone as a transmission signal to a car telephone radio, a reception system output from the radio and output as voice from a speaker, and the transmission system. And an adaptive digital filter as an acoustic echo canceller inserted between the receiving system and
An adder that adds the transmission signal and the output of the adaptive digital filter; a first adaptive high-pass filter that is inserted in the transmission system before the adder; and the reception system and the adaptive digital filter. And a second adaptive high-pass filter interposed between the first and second adaptive high-pass filters, the characteristics of which are adaptively changed based on the output level of the adder. Adaptive noise reduction car telephone system. 5. A first arithmetic unit for calculating an effective value of a signal input to a second adaptive filter, and a second arithmetic unit for calculating an effective value of a signal output from the second adaptive filter. A comparison arithmetic unit for performing a comparison operation on the outputs of the first and second arithmetic units to calculate a multiplication coefficient; and a multiplication inserted in the reception system for controlling the level of the reception signal based on the multiplication coefficient. 5. The adaptive noise reduction car telephone system according to claim 4, further comprising: 6. The adaptive noise-removing car telephone device according to claim 1, which is configured as a hands-free car telephone device.